The present invention relates generally to Doppler shifted radar and more specifically to target identification with respect to surveillance of moving vehicles. More particularly, it relates to an improved system using two or more continuously transmitting frequencies directed toward target vehicles whereby the phase difference of the reflected Doppler signals is scaled to a target range. As such the system can detect the speed of each target within a specified range including the closest, the next closer, next faster, fastest closing vehicle, etc.
One of the most common and useful tools in the enforcement of vehicular speed limit laws has been the use of Doppler radar. In a Doppler radar system, a microwave signal is transmitted to a vehicle and then reflected off the vehicle. When the reflected signal is received back at the Doppler radar system, a change of frequency in the signal is proportional to the vehicle speed. That shift in signal frequency is known as the xe2x80x9cDoppler Effectxe2x80x9d. The shift in frequency is measured and the resulting vehicle speed is calculated and displayed on the radar system, thereby eliminating any need for calculations for the system operator (police officer) in the vehicle that has the radar system. The radar system may be permanently mounted in the vehicle or the system may be a portable hand-held unit with a display. If a vehicle speed is measured over the speed limit, the officer may pursue the vehicle and issue a speeding ticket.
In actual application, a problem arises when more than one vehicle is in the visual sight of the operator. The problem is the identification of which vehicle""s speed is displayed when more than one vehicle is in a position to reflect the microwave signal. Conventional radar is always instructed to display the speed of the xe2x80x9cstrongestxe2x80x9d radar return in the xe2x80x9cstrongest target windowxe2x80x9d, most often the leftmost window of the radar display. Since the round trip strength of the radar signal drops off at 1/R4 (where R is the distance to the target), it can be somewhat assured that the strongest return is also comparable to the closest target. The using system operator will develop a tracking history (speed values over time) and mentally correlate the reading from the actual viewed traffic. When a speed violator has been detected and confirmed (through the tracking history), then the operator may issue a citation.
The originating signal is usually conical shaped and emits a half angle of six degrees. As the range is increased, more vehicles can come within the xe2x80x9cmeasurement conexe2x80x9d and reflect back to the system. The best reflector of the radar is a function of the vehicle design, that is, vehicle size, shape, grills, headlights, bumpers, large flat surfaces, etc. When a plurality of vehicles with various geometric shapes enters a radar area, there may be a situation where the identification of a target vehicle is indeterminate. All of these factors contribute to the problem of reliable vehicle identification.
When citizens come into contact with law-enforcement personnel, it is important that they feel that they have been dealt with in a fair and consistent manner. Thus, it is important that traffic citations be based on factual matter and not be left to any discretion of the law-enforcement officer. Police traffic radar has always suffered from the inability to accurately report the range of, a target object. It has always been taught to system operators that the strongest return corresponds to the closest target. Unfortunately, this only applies to vehicles of the same size, shape, etc. Vehicles that vary significantly in their geometric properties can produce equally varying signal strengths. Common roadway traffic is usually composed of a multitude of vehicle sizes, shapes, etc. ranging from motorcycles, sports cars to large eighteen-wheeler trucks. These varying targets increase the difficulty of correctly identifying the closest vehicle based on signal strength alone.
There are many methods to detect the relative or exact range of target vehicles. These methods include pulsing, modulation of the carrier wave and alternately transmitting more than one carrier frequency towards a target. The pulsing methods have drawbacks with FCC limitations and the modulation methods tend to be more complex and thus more expensive.
The present invention will describe a means that accurately determines the exact or relative range to a target while being relatively inexpensive, stable, and accurate over time and temperature variations. The present invention resolves issues of accurate target identification regarding the radar operator use of discretion when operating the radar system. The present invention will provide a means for the radar operator to correctly identify the closest target in range thereby substantially improving target identification. The present invention utilizes two or more xe2x80x9ccontinuouslyxe2x80x9d transmitting frequencies directed toward the moving target(s) whereby the phase difference of the reflected Doppler signals is scaled to a target range.
The main aspect of the present invention is to provide for an improved vehicle radar system whereby two or more continuous transmitting frequencies provide for correct identification of the closest target in range, thus improving target identification.
Another aspect of the present invention is to provide the relative range of a target by comparing the phase differences between the Doppler signals.
Another aspect of the present invention is to provide for a longer detection range.
Another aspect of the present invention is to provide for better stability and accuracy over the lifetime of the radar unit.
Another aspect of the present invention is a relatively inexpensive manufacturing cost and compact size.
Another aspect of the present invention is the ability to combine the range and speed.
Another aspect of the present invention, an alternate embodiment, is to provide a speed for a plurality of relative targets coinciding with each targets range.
Another aspect of the present invention, an alternate embodiment, is to report and/or display the relative speeds of the closest, next closest, etc. vehicles.
Another aspect of the present invention is to allow the operator of the radar system to control the display to show the speed of the closest, then switch to the next closest or again next closest vehicle as desired.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.
The present invention employs two or more continuous frequencies and is thus a multiple frequency transmitting radar system (MFTRS). The MFTRS can determine the relative range to a target by comparing the phase differences between the returned Doppler signals. This phase difference is directly proportional to the range of the target. Determination of the relative range of a vehicle will then allow the MFTRS to display the speed of the closest vehicle.
The use of continuous transmission of two or more frequencies overcomes many problems with prior art, which used alternating transmitting signals, thereby introducing errors into a system. Further detail of the MFTRS will be described below.
The MFTRS provides continuous transmitting frequencies for correct identification of the closest target in range (and its speed), thus improving target identification. The transmitting frequencies are phase locked to a reference offset to provide a stable output over time and temperature. The MFTRS utilizes the combining action of these continuous transmission signals and eliminates many errors commonly found in prior art. A combiner circuit takes the transmitted frequencies from two separate microwave sources and combines (adds) them together before being transmitted. The combiner circuit will accurately perform the task while offering low insertion loss and is relatively inexpensive to produce. The use of a combiner allows that the transmitting frequencies originate from one antenna, which has been determined to be a requirement (confirmed by experimental results) due to the Cosine effect, which will be discussed in detail below. Also, with continuous transmitting frequencies, there is far less chance that the frequencies will interfere with other equipment as compared to prior art alternating frequencies.
Prior art have alternately transmitted two or more microwave frequencies from one antenna. That is, radar will transmit one frequency for a brief time and then switch to another alternate frequency. Basic error improvements over the aforementioned methods of prior art include:
A. Cosine effectxe2x80x94The cosine effect causes a radar unit to display a speed, which is lower than the actual target speed. This condition exists when the target vehicle""s path is not parallel to the antenna, such as on a curve or hill. As the angle between the radar beam and the target increases, the displayed speed decreases. It is imperative that the Doppler signals from moving targets be compared to each other as simultaneously as possible. That is, readings should be as synchronous as possible. In prior art two or more microwave frequencies have been xe2x80x9calternatelyxe2x80x9d transmitted. The radar system will transmit one frequency for a brief time, (on the order of micro-seconds) and then switch to an alternate frequency for a brief time, then switch back and so on. When one transmitter varies frequency from one frequency to another, xe2x80x9cgapsxe2x80x9d will exist from one time element of a frequency transmission to that of another frequency transmission. To prevent loss of synchronization, the alternating rate (switching time from one frequency to the other) needs to be very fast, usually on the order of microseconds. Such a high alternating rate can create errors in the transmitters by forcing the microwave oscillator to jump from one frequency to another. Typically, there exists a time space xe2x80x9cdead timexe2x80x9d to allow stabilization prior to processing. The MFTRS eliminates these issues with the cosine effect since each oscillator is transmitting continuously, and both Doppler signals are sampled at exactly the same instant in time. Thus, there is no xe2x80x9cdead timexe2x80x9d.
B. Range reductionxe2x80x94It is desirable to have good range detection on a police traffic surveillance radar system. Most traffic officers base the quality of the radar system on its range ability. That is longer detection range is equated to better-perceived quality. As in prior art, the alternating of frequencies results in less energy being recovered from the transmitted signal, which reduces the overall detection range. The frequency-alternating rate, as described in xe2x80x9cAxe2x80x9d above, the dwell time (time on the target) is reduced. This short dwell time reduces the time that the return signal has to overlap and xe2x80x9cmixxe2x80x9d with the reference frequency of the source oscillator. This short overlap, in turn, reduces the energy that is received and thus reduces the signal to noise ratio of the returned signal. The MFTRS of the present invention eliminates these dwell time issues as the frequencies are continuous and thus there are no dwell time or overlap issues. In addition, both received Doppler""s can be averaged together to increase the target detection range.
C. Stabilityxe2x80x94With prior art, since the frequencies are alternating, there are no safe guards to insure that the frequencies maintain the correct offset over time and/or temperature changes. With the MFTRS of the present invention, employment of continuous frequencies does not have these limitations. The continuous frequencies of the MFTRS can be frequency locked together to provide stability and accuracy over the lifetime of the MFTRS.
The following discussion relates to the theory of For a dual transmitting device the maximum unambiguous range is given as:
Ur=C/(2*xcex94f)
Ref: RADAR SYSTEM DESIGN and ANALYSISxe2x80x9d by S. A. Hovanessian copyright 1984 ARTECH HOUSE, INC. Chapter 4 xe2x80x9cMathematical Derivation of Radar Equationsxe2x80x9d, Pgs 84, 85
xe2x80x83Where: Ur is the unambiguous range;
C is the speed of light (186,282 miles/sec);
xcex94f is the frequency of separation between the two transmitted frequencies; and
xe2x80x9c*xe2x80x9d is a multiplication symbol.
Assuming that a frequency of separation, xcex94f, is 200 KHz, the maximum unambiguous range, Ur, would then be approximately 2459 feet. Decreasing xcex94f to 100 KHz would double Ur to give close to a mile of unambiguous range. The MFTRS transmits the two frequencies and computes the phase difference of the returned Doppler signals to calculate the target range. The phase difference is measured and scaled to a target range. Obviously, increasing the difference frequency improves the resolution of the detected range, however the unambiguous range is decreased.
The received Doppler signals are processed via a xe2x80x9cFast Fourier Transformxe2x80x9d (FFT). The FFT produces a spectrum that ranges from zero Hz in frequency (DC value) upward in frequency limited to the sampling rate divided by two. A typical sampling rate is 46,875 Hz for Ka-band operation and 31,250 Hz for K-band operation. The sampling rate is not important as long as the rate is twice that of the highest frequencies expected.
The target vehicle speed is calculated from the frequency spectrum by using the Doppler shift for that particular band. The K-band frequency of operation is 24.050 GHz to 24.250 GHz and the Ka-band frequency of operation is 33.4 GHz to 36.0 GHz. The Doppler shifts are about 72.038 Hz per mile per hour for the K-band (centered on the bandxe2x88x9224.150 GHz) and about 105.9 Hz per mile per hour for the Ka-band (centered at 35.5 GHz). The MFTRS can operate using either K-band or Ka-band depending on the end user requirements. Ka-band would be the preferred operational band.
For example, if the antenna is transmitting at the Ka-band and a vehicle is traveling at 100 mph its Doppler shift will be 105. 9 Hz/mph multiplied by 100 mph or 10590 Hz frequency shift.
The phase information of each target vehicle is revealed by the imaginary and real values of the FFT at that particular target location. The phase difference information is calculated by:
"PHgr"1=Tanxe2x88x921 Imagch1/Realch1
"PHgr"2=Tanxe2x88x921 Imagch2/Realch2
Phase Difference=xcex94"PHgr"=|xcfx861xe2x88x92xcfx862|
The phase difference information is calculated for each target and used to classify the relative distance (range) between the MFTRS and the target vehicle. The range is calculated with the equation:
Rcalc=(c*xcex94"PHgr")/(4Π*xcex94f)
Ref: RADAR SYSTEM DESIGN and ANALYSISxe2x80x9d by S. A. Hovanessian copyright 1984 ARTECH HOUSE, INC. Chapter 4 xe2x80x9cMathematical Derivation of Radar Equationsxe2x80x9d, Pgs 84, 85
Where Rcalc is the calculated target range;
c is the speed of light;
xcex94"PHgr" is the phase difference between the
two Doppler signals; and
xcex94f is the frequency of separation (or frequency difference) between the two transmitted frequencies.
The use of two frequencies thus provides the ability to calculate the range of each target and determine the closest target thereby displaying correct identification of a target speed and overcoming prior art, which displayed the strongest target (which is not necessarily the closest target as previously discussed). A typical minimum frequency difference is about 100 kHz.
Once all of the potential targets have been classified, they are sorted based on the range (phase information) and then sorted from closest to farthest for display by the MFTRS. Although the preferred embodiment of the present invention is to display the xe2x80x9cclosestxe2x80x9d vehicle speed, alternate embodiments can easily display a plurality of vehicle speeds starting with the xe2x80x9cclosestxe2x80x9d then the xe2x80x9cnext closestxe2x80x9d and so on. In addition to phase information, amplitude information and frequency information is also saved for each of the targets detected. It should be noted that increasing the number of data points that are taken could increase the resolution of the target speed and phase information. Increasing the FFT size can improve the frequency resolution of each target.
Although only one frequency is required to calculate target speed, the second of the two transmitted frequencies can also provide a calculation as to the second Doppler speed from the same target. This xe2x80x9csecondxe2x80x9d speed can be used by the MFTRS to self-detect any out of specification conditions.
The MFTRS apparatus of the preferred embodiment of the present invention is compact as is described in the following description. The MFTRS contains two antennae, one for the front and one for the back of the user vehicle. The MFTRS contains a computer and display unit, which can be separated for space saving if required. The computer and display unit interconnect with a simple 9-pin connector (and 9-pin cable if physical separation is desired). The computer/display, or display unit alone, can easily be mounted on a dash for easy readability. The hand remote controller connects to the computer unit via a quick disconnect tethered cable. A power connector also plugs into the computer unit at one end and an auxiliary cigarette lighter plug on the other end. Each front and rear antenna also connects into the computer unit. The handheld remote controller activates either the front or the rear antenna (it is also possible to only mount one antenna). A serial communications port (COM) is also provided to allow the MFTRS to communicate with other devices such as PC""s, speed signs, in-car video systems, etc. It should be noted that although a tethered remote control is described above as the preferred embodiment, a non-tethered remote control could also be used.
The display section of the preferred embodiment of the MFTRS would contain four windows for the operator display:
1. The first (leftmost) target speed window would contain the xe2x80x9cclosestxe2x80x9d or target vehicle speed. Underneath the first window could be a set of LED""s to show the relative range of the vehicle. That is, the number of LED""s lit would be a function of the target vehicle""s relative range.
2. The second window would be a locked window. The operator would enter a xe2x80x9clockxe2x80x9d key on the handheld remote to lock a target speed into the xe2x80x9clock speedxe2x80x9d window, which can be later xe2x80x9cclearedxe2x80x9d by operator entry. Under the lock window there are two LED""s to indicate whether the xe2x80x9cfrontxe2x80x9d or xe2x80x9crearxe2x80x9d antenna is transmitting.
3. The third window would be a xe2x80x9cmodexe2x80x9d window to display the mode of operation. It would show stationary mode, moving mode same direction, moving mode opposite direction. This window would also be used to display error messages such as handheld remote not connected (RMT?), low voltage (LowV), system failure (SYS), etc.
4. The fourth window is used to display the patrol car speed. This window would be used in moving mode to give the operator a visual check with the patrol car speedometer to offer assurance that the MFTRS is functioning properly.
It should be noted that the order in which the windows are manufactured could be changed as required.
Again, although the preferred embodiment of the MFTRS would display only the closest vehicle speed, alternate embodiments of the MFTRS could easily visually display a series of target data if required. There are many combinations of alternate embodiments that can be employed in the packaging of the MFTRS. For example, the MFTRS can have a plurality of windows to display target xe2x80x9conexe2x80x9d as the closest target, displaying its range (if desired) and speed, target xe2x80x9ctwoxe2x80x9d as the next closest target, displaying target two""s range (if desired) and speed, then display target xe2x80x9cthreexe2x80x9d with its data, then target xe2x80x9cfourxe2x80x9d and so forth.
Within a plurality of display windows, another alternate embodiment would be to have the display programmed to highlight the xe2x80x9cfastestxe2x80x9d target within a certain range (1000 feet, for example) displayed and its order of occurrence in the visual field. Yet another embodiment could have a button that had xe2x80x9cnext fasterxe2x80x9d on it and, upon pressing this button, the radar would display the next target, which is the next target which is faster. Another embodiment could employ, for example, a flash frequency, then highlight the xe2x80x9cnext fasterxe2x80x9d target with a slower flash frequency.
If a display were as the aforementioned preferred embodiment with four windows, an alternate embodiment of the display could also be set up to show the speed of the closest vehicle and allow the MFTRS operator to switch the display to either the xe2x80x9cnext closestxe2x80x9d or to the xe2x80x9cnext fasterxe2x80x9d of the vehicles within range. There could also be an operator input to allow the speed display (leftmost window) to indicate only approaching or moving away vehicles to separate roadway traffic in the display.
Range options for alternate embodiments could include actual range display (in feet, miles, meters, etc.) or display a moving bar (bar would decrease in length as the vehicle approached or increase in length as the vehicle moved away). Other options would be to display an arrow to indicate the moving direction of the target (approaching or moving away). Although the actual range could be displayed, it is felt that providing the operator with too many numerical options would only lead to confusion if displaying a multiple of variables.
The MFTRS provides a relative targets"" speed. Again, it can include range, a bar indicator, or directional arrow if desirable. The MFTRS can also link with a visual display apparatus (monitor, camera, etc.) in order to link and attach the range/speed with each target. The MFTRS can be packaged as a stationary mount with an operator vehicle or as a handheld device.
The computer unit contains all of the electronics including the digital signal processor (DSP), memory, oscillators, regulators, amplifiers, filters, isolators, circulators, connectors, etc. The logic will be described below. The connectors include a power input connector, front and rear antenna connectors, a communication port and a connector to the remote handheld.
The handheld remote controller is connected to the computer unit. It would comprise of a small built-in speaker and have the following buttons:
a. Power (PWR) button for activating the MFTRS.
b. Front antenna and a rear antenna buttons (Up/Down) to activate or deactivate the front and rear antenna. Only one antenna is allowed to be in the active state at one time.
c. A target lock buttonxe2x80x94when hit it will transfer the target speed from the xe2x80x9cclosestxe2x80x9d window and lock it into the xe2x80x9clockxe2x80x9d window. The xe2x80x9cclosestxe2x80x9d window would continue to track the target speed;
d. A mode button to switch between stationary mode, moving mode opposite-direction, and moving mode same-direction.
e. A test button for self-test and diagnostics of the MFTRS.
f. A squelch button to allow the user to select the type of Doppler audio sound heard. The user can select the target""s Doppler or other sounds.
g. A toggle range button (+/xe2x88x92) to allow the user to decrease or increase the target acquisition distance.
h. A toggle volume (+/xe2x88x92) button to allow the user to control the Doppler audio volume and the system xe2x80x9cbeepxe2x80x9d volume.
i. A City/Highway option button to allow the MFTRS to track minimum slow versus fast speed. The speed limitations can be set depending on user requirements.
j. An optional fast button to allow the user to track the next faster target.
It should be noted that, in the aforementioned description, the remote handheld is for purposes of illustration and that there are many combinations and layouts of a handheld that would function to provide adequate MFTRS operation.
General Operation:
1. Power Up the Unitxe2x80x94a self test starts and will display a Test Pass or a System Error in the mode window.
2. Upon power up the mode will contain an xe2x80x9cAnt?xe2x80x9d asking the user to select the front or rear antenna from the handheld buttons. Once an antenna is selected, an LED will indicate which antenna has been selected.
3. The mode window will show which mode is selected:
a. Stationaryxe2x80x94a bar will show that the MFTRS is in stationary mode and one of two arrows will show the target direction (toward or away).
b. Moving mode opposite directionxe2x80x94a leftmost downward arrow and a rightmost upward arrow indicates the moving mode opposite direction.
c. Moving mode same directionxe2x80x94the mode window will display two upward arrows indicating both vehicles moving in the same direction.
4. Pushing the lock button will transfer a target speed into the target lock window while continuing to monitor the target speed in the target speed window.
5. Clear the lock as required by hitting the lock button when the antenna is transmitting, changing the mode, or turning the antenna off and on again.
While the above is a simple description of the apparatus and operation thereof, there are other embellishments of the display, handheld, etc. that can be manufactured depending on user requirements.
The drawings to follow will show the circuitry and logical flow of signal and signal processing components.